“Welcome Back Kotter” star Ron Palillo, who died Tuesday at 63, shined playing a misfit who fit in perfectly with his fellow Sweathogs.

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The "Welcome Back, Kotter" cast in 1978. Ron Palillo, center, who played Arnold Horshack, died at his Palm Beach, Florida home Tuesday.

Even if you never watched “Welcome Back, Kotter” during its mid-1970s hey-day or in three-plus decades of syndication, you knew Arnold Horshack.

He was the high school oddball with the screechy voice, the one with his hand thrust and bobbing in the air, yelling “Ooh! Ooh! Ooh!” to get the teacher’s attention – only to crack wise when called upon. There was a Horshack in every classroom – and, eventually, on a fair number of TV shows, thanks to talented comic actor Ron Palillo, who died Tuesday at age 63. His charmingly annoying class-clown character became the sitcom forerunner to the much-smarter Urkel and Horshack’s super-grating “Saved by the Bell” bastard son, Screech.

Arnold Horshack emerged as a TV archetype and memorable character in his own right via Palillo, who shined playing a misfit who fit in perfectly with his fellow loser Sweathogs on a popular show that was much more than just John Travolta’s launching pad to fame.

For the uninitiated, “Welcome Back, Kotter” starred comedian Gabe Kaplan as a teacher who returns to his old, tough Brooklyn high school on a kind of Groucho Marx-meets-“Blackboard Jungle” mission to help remedial cases on the verge of expulsion. A former Sweathog himself, Kaplan’s Kotter wielded Borsht Belt humor as tool for teaching history and life lessons.

Travolta played Vinnie Barbarino, the seemingly dull-witted stud of James Buchanan High whose favorite insult, “Up your nose with a rubber hose,” became a national catchphrase (you had to be there). Lawrence Hilton-Jacobs portrayed the smooth, too-cool-for-school Freddie “Boom-Boom” Washington, who was armed with a Barry White-deep voice that greeted all comers with his own catchphrase, “Hi there.”

Robert Hegyes played Juan Epstein, the Chico Marx wannabe with endless excuses, via notes from home signed, “Epstein’s Mother.” Palillo’s Horshack, with his curly hair and vagabond attire, looked like Harpo Marx – except that he almost never stopped talking, endlessly introducing himself in a nasally, old-school Brooklynese. “Hello. How are ya? I’m Arnold Horshack,” he’d say, turning the last syllable of his last name into an elongated snort.

For those of us who grew up watching sitcoms in the 1970s, the Sweathogs were the somewhat grittier New York alternative to the very different high school life depicted on the Milwaukee-set “Happy Days,” which also aired on ABC. “Kotter,” which lasted only four seasons from 1975 to 1979, stands as a still-entertaining TV time capsule with its classic opening theme song and scenes-of-Brooklyn sequence – not mention its look at Travolta just before, and for a short while after, he exploded onto movie screens in “Saturday Night Fever.”

While Travolta became a superstar and Kaplan launched a second career as a top poker player, the other cast members never approached their “Kotter” success after the show went off the air. Hegyes, sadly, died in January at age 60. Palillo likely spent much of the last decade regretting his decision to go on Fox’s old “Celebrity Boxing” show, which ended in his pummeling by Dustin “Screech” Diamond in a match conceived in some Freudian hell.

We prefer to remember Palillo as the smart aleck whose hand in the air heralded not a fist to fly, but a laugh to come, on a show that, like an old friend, is always welcome.